Small intestine

The small intestine is the longest (>6 m) and most convoluted organ in the digestive system. It is divided into three segments:

Most digestion and absorption take place in the small intestine, the mucosa of which is well adapted for these functions with certain anatomical modifications:

• Plicae circulares

• Microvilli

The plicae circulares, or circular folds, form internal rings around the circumference of the small intestine that are found along the length of the small intestine. They are formed from inward foldings of the mucosal and submucosal layers of the intestinal wall. The plicae circulares are particularly well developed in the duodenum and jejunum and increase the absorptive surface area of the mucosa about threefold. Each plica is covered with millions of smaller projections of mucosa referred to as villi. Two types of epithelial cells cover the villi:

• Absorptive cells

The goblet cells produce mucus. The absorptive cells, found in a single layer covering the villi, are far more abundant. Taken together, the villi increase the absorptive surface area another 10-fold.

Microvilli are microscopic projections found on the luminal surface of the absorptive cells. Each absorptive cell may have literally thousands of microvilli forming the brush border. These structures increase the surface area for absorption another 20-fold. Together, these three anatomical adaptations of the intestinal mucosa — plicae circulares, villi, and microvilli — increase the surface area as much as 600-fold, which has a profound positive effect on the absorptive process.

Motility of the small intestine. Segmentation and peristalsis take place in the small intestine. Segmentation mixes chyme with digestive juices and exposes it to the intestinal mucosa for absorption. This form of motility causes only a small degree of forward movement of the chyme along the small intestine. Peristalsis, the wave-like form of muscle contraction, primarily moves chyme along the intestine and causes only a small amount of mixing. These contractions are weak and slow in the small intestine so that time is sufficient for complete digestion and absorption of the chyme as it moves forward. Intestinal peristaltic contractions are normally limited to short distances.

Segmentation contractions occur as a result of the basic electrical rhythm (BER) of pacemaker cells in the small intestine. This form of muscular activity is slight or absent between meals. The motility of the small intestine may be enhanced during a meal by:

• Distension of the small intestine

• Extrinsic nerve stimulation

During a meal, segmentation occurs initially in the duodenum and the ileum. The movement of chyme into the intestine and the distension of the duodenum elicit segmentation contractions in this segment of the small intestine. Segmentation of the empty ileum is caused by gastrin released in response to distension of the stomach. This mechanism is referred to as the gastroileal reflex. Parasympathetic stimulation, by way of the vagus nerve, further enhances segmentation. Sympathetic stimulation inhibits this activity.

Digestion and absorption in the small intestine. Most digestion and absorption of carbohydrates, proteins, and lipids occurs in the small intestine. A summary of the digestive enzymes involved in these processes is found in Table 18.3.

Carbohydrates. Approximately 50% of the human diet is composed of starch. Other major dietary carbohydrates include the disaccharides, sucrose (table sugar, composed of glucose and fructose) and lactose (milk sugar, composed of glucose and galactose). Starch is initially acted upon by amy-lase. Salivary amylase breaks down starch molecules in the mouth and stomach. Pancreatic amylase carries on this activity in the small intestine. Amylase fragments polysaccharides into disaccharides (maltose, composed of two glucose molecules). The disaccharide molecules, primarily maltose, are presented to the brush border of the absorptive cells. As the disaccharides are absorbed, disaccharidases (maltase, sucrase, and lactase) split these nutrient molecules into monosaccharides (glucose, fructose, and galactose).

Glucose and galactose enter the absorptive cells by way of secondary active transport. Cotransport carrier molecules associated with the disaccha-ridases in the brush border transport the monosaccharide and a Na+ ion from the lumen of the small intestine into the absorptive cell. This process is referred to as "secondary" because the cotransport carriers operate passively and do not require energy. However, they do require a concentration gradient for the transport of Na+ ions into the cell. This gradient is established by the active transport of Na+ ions out of the absorptive cell at the basolateral surface. Fructose enters the absorptive cells by way of facilitated diffusion. All monosaccharide molecules exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries.

Proteins. Protein digestion begins in the stomach by the action of the gastric enzyme pepsin. This enzyme fragments large protein molecules into smaller peptide chains. Digestion is continued in the small intestine by the pancreatic enzymes trypsin, chymotrypsin, and carboxypeptidase, which hydro-lyze the peptide chains into amino acids, dipeptides, and tripeptides. Similar to glucose and galactose, amino acids enter the absorptive cells by way of secondary active transport. Once again, energy is expended to pump Na+ ions out of the absorptive cells, creating a concentration gradient for the cotransport of amino acids and Na+ ions into the cell.

Note: The role of lingual lipase in the digestion of dietary lipids is minor because it accounts for less than 10% of the enzymatic breakdown of triglycerides.

Dipeptides and tripeptides are also presented to the brush border of the absorptive cells. As the nutrient molecules are absorbed, aminopeptidases split them into their constituent amino acids. The activity of aminopeptidases accounts for approximately 60% of protein digestion. The amino acid molecules then exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries.

Lipids. Dietary fat consists primarily of triglycerides. Fat digestion begins in the mouth and stomach by the action of the salivary enzyme lingual lipase. However, the role of this enzyme is minor because it accounts for less than 10% of enzymatic breakdown of triglycerides. Lipids are digested primarily in the small intestine. The first step in this process involves the action of bile salts contained in the bile. Bile salts cause emulsification, which is the dispersal of large fat droplets into a suspension of smaller droplets (<1 mm). This process creates a significantly increased surface area upon which fat-digesting enzymes can act.

Because intact triglycerides are too large to be absorbed, pancreatic lipase acts on the lipid droplets to hydrolyze the triglyceride molecules into monoglycerides and free fatty acids. These water-insoluble constituent molecules would tend to float on the surface of the aqueous chyme; therefore, they need to be transported to the absorptive surface — a process carried out by micelles formed by the amphipathic bile salts. The bile salts associate with each other such that the polar region of the molecule is oriented outward, making them water soluble. The nonpolar region faces inward away from the surrounding water; the monoglycerides and free fatty acids are carried in this interior region of the micelle. Upon reaching the brush border of the absorptive cells, they leave the micelles and enter the cells by simple diffusion. Because they are nonpolar, these molecules move passively through the lipid bilayer of the cell membrane. This process takes place primarily in the jejunum and proximal ileum. The bile salts are absorbed in the distal ileum by way of passive diffusion or secondary active transport.

Within the absorptive cells, the monoglycerides and free fatty acids are transported to the endoplasmic reticulum, which contains the necessary enzymes to resynthesize these substances into triglycerides. The newly synthesized triglycerides then move to the Golgi apparatus. Within this organelle, they are packaged in a lipoprotein coat consisting of phospholip-ids, cholesterol, and apoproteins. These protein-coated lipid globules, referred to as chylomicrons, are now water soluble. Approximately 90% of the chylomicron consists of triglycerides.

Chylomicrons leave the absorptive cell by way of exocytosis. Because they are unable to cross the basement membrane of the blood capillaries, the chylomicrons enter the lacteals, which are part of the lymphatic system. The vessels of the lymphatic system converge to form the thoracic duct that drains into the venous system near the heart. Therefore, unlike products of carbohydrate and protein digestion that are transported directly to the liver by way of the hepatic portal vein, absorbed lipids are diluted in the blood of the circulatory system before they reach the liver. This dilution of the lipids prevents the liver from being overwhelmed with more fat than it can process at one time.

Water and electrolytes. Each day in an average adult, about 5.5 l of food and fluids move from the stomach to the small intestine as chyme. An additional 3.5 l of pancreatic and intestinal secretions produce a total of 9 l of material in the lumen. Most of this (>7.5 l) is absorbed from the small intestine. The absorption of nutrient molecules, which takes place primarily in the duodenum and jejunum, creates an osmotic gradient for the passive absorption of water. Sodium may be absorbed passively or actively. Passive absorption occurs when the electrochemical gradient favors the movement of Na+ between the absorptive cells through "leaky" tight junctions. Sodium is actively absorbed by way of transporters in the absorptive cell membrane. One type of transporter carries a Na+ ion and a Cl- ion into the cell. Another carries a Na+ ion, a K+ ion, and two Cl- ions into the cell.

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.